Olefin separation process

ABSTRACT

A mixture of olefins and n-paraffins is produced by cracking or dehydrogenation of n-paraffins. Separation of an olefin fraction from the mixture of olefins and normal paraffins is effected by forming a solution of said mixture in a selective solvent, cooling said solution to a temperature at which substantially all of said olefins remain in solution in said solvent whereby normal paraffins are precipitated from the remaining solution. The remaining solution of enriched olefin content is separated from precipitated normal paraffins and solvent and to said olefin fractions are separated from the normal paraffin denuded liquid. The separated olefin fraction is suitable for reaction with an aromatic such as benzene in the manufacture of alkyl benzene compounds.

United States Patent Parker et al.

[451 Sept. 12,1972

[54] OLEFIN SEPARATION PROCESS 3,235,471 2/1966 Clay ..260/677 AInventors: Levi Parker; Th Allen Woodie C ames Lawson M do 5 all ggs 13ea w Primary Examiner-Curtis R. Davis Attorney-Thomas H. Whaley and CarlG. Ries [73] Assignee: Texaco Inc.,New York,N.Y. 22 Filed: Dec. 28, 1970[571 ABSTRACT [21] Appl. No.: 101,889 A mi tture of olefins andn-paraffins is produced by cracking or dehydrogenation of n-paraffins.Separation of an olefin fraction from the mixture of olefins [52] US. Cl..260/671 R, 260/671 B, 260/677 A, and normal paraffins is effected byforming a Solution 260/683'3 of said mixture in a selective solvent,cooling said [51] Int. Cl. ..C07c 3/52, C07c 11/02 Sohnion to atemperauure at which substantiauy a of [58] new of sea'ch-m-260/677 671676 said olefins remain in solution in said solvent whereby R; 208/33normal paraffins are precipitated from the remaining solution. Theremaining solution of enriched olefin [56] References C'ted content isseparated from precipitated normal paraffins and solvent and to saidolefin fractions are ATES PAT NT UNITED ST E S separated from the normalparaffin denuded liquid. 2,911,452 1959 Bl'oughton --260/677 A Theseparated olefin fraction is suitable for reaction Klnchen an aromaticuch as benzene in the manufacture Pokomy et al. of benzene compounds2,925,452 2/1960 Broughton ..260/677 A Claims, 1 Drawing Figure 280F/P/ICT/fl/VA T0,? 2/ J0 Df/lVfl/FflfiE/VATOI? ail F/Z 7E1? 7 1:JIfi/flPf/F Jffi/Pfi'l? :7 L azzi/n/ Ja\ P/FMUCT 40 i .19 ALA/V1470?OLEFIN SEPARATION PROCESS BACKGROUND OF THE INVENTION Long chain normalolefins, for example, olefins having chain links of at least carbonatoms are valuable in the manufacture of petrochemically derivedproducts, for example, biodegradable detergents. Such materials are ingreat demand as a means of reducing the pollution of streams and watersources. An economical and effective method of producing normal olefinsis by wax cracking or the dehydrogenation of normal paraffins. Suchprocesses are described in U.S. Pat. No. 2,172,228 which relates to athermal cracking method and US Pat. No. 3,458,592 which describes acatalytic method of producing n-olefins from n-paraffins. In suchprocesses, the product comprises n-olefins in admixture with unconvertedn-paraffins. In processes with the greatest selectivity, that is,processes giving the greatest yield of n-olefin based on n-paraffinconversion, conversion per pass is relatively low, and the olefinproduct is diluted with unconverted n-paraffin charge. When employingthe n-olefin product as feed to an alkylation process for themanufacture of products such as detergent alkylate, it is desirable toconcentrate the olefin feed to reduce diluents in the alkylation systemand to avoid the production of undesired byproducts. Accordingly, it isan object of this invention to provide a means of separating an olefinfraction of increased olefin content from a mixture of olefins andnormal paraffins. It is a further object to concentrate the olefinsproduced in a n-paraffin cracking process to provide a feedstocksuitable for alkylation.

SUMMARY OF THE INVENTION This invention is directed to a process ofseparating an olefin fraction from a mixture of olefins and n-paraffinswhich comprises forming a solution of said mixture in a selectivesolvent, cooling said solution to a temperature at which saidn-paraffins are substantially insoluble in said solvent and at whichsubstantially all of said olefins remain in solution in said solventeffecting precipitation of nparaffins from remaining solution ofincreased olefin concentration, separating precipitated n-paraffins fromsaid remaining liquid, and separating solvent and said olefin fractionfrom said remaining liquid. The selective solvent employed in thisinvention may comprise, for example, acetone, methylethyl ketone,methylisobutyl ketone, benzene, toluene, naphtha, gasoline, pentane,dichloromethane, methyl chloride, chloroform, and mixtures thereof. Aparticularly preferred solvent is a mixture of an aliphatic ketone andan aromatic hydrocarbon, for example, a mixture of methylethyl ketoneand toluene. In accordance with this method, the solution of olefins,nparaffins and solvent is cooled to a temperature effectingprecipitation of the normal paraffins as a waxy solid from a solutioncomprising olefins and solvent. The temperature of normal paraffinprecipitation depends upon the carbon number of the paraffin orparaffins involved. It is preferred to employ a separation temperatureof about the melting point of the corresponding olefin. For example, inthe separation of a mixture of olefins from the corresponding paraffins,we prefer to employ separation temperature of about the following: irle, hi urm ut- 19,

+; G -C C -C +100. Desirably the feedstock to the separation process ofthis invention is distilled to separate it into cuts of olefins andn-paraffins having carbon numbers differing by not more than two carbonnumbers. The separated olefin fraction is suitable for use in alkylationwherein it is contacted with benzene in the presence of an alkylationcatalyst under alkylation conditions effecting formation of alkylbenzenes. Advantageously, in a combination process wherein a normalparaffin charge stock is dehydrogenated forming said mixture of olefinsand nparaffins and the olefin product is separated by precipitatingn-parafiins, the precipitated n-paraffins are recycled to comprise aportion of the normal paraffin charge stock.

In the n-paraffin dehydrogenation process, the olefin content of then-parafiin-olefin effluent may typically range from 10 to 15 volumepercent. It is desirable to separate these olefins from the effluent, inorder to use the olefins for further processing, and return thenparaffins to the dehydrogenation process for further conversion toolefins. Alternatively, an enrichment of the olefin-fraction in then-paraffin-olefin effluent is also desirable as an enrichedolefin-n-paraffin mix may be employed as charge stock for furtherprocessing. For example, an enriched fraction of olefin and nparaffinmay be charged with benzene to an alkylation unit for production ofdetergent alkylate. Unreacted nparaffins from detergent alkylatemanufacture may also be returned as charge stock to the n-paraffindehydrogenation unit.

We have found that by using a solvent-chilling technique, thatn-paraffins can be removed as a waxlike precipitate from ann-paraffin-olefin dehydrogenation effluent providing an olefin stream ofenriched olefin content. The solvent-chilling technique is similar tothat used for removing wax from lubricating oils.

DESCRIPTION OF THE DRAWING The accompanying drawing diagrammaticallyillustrates the process ofthis invention. A charge stream comprisingn-paraffin hydrocarbons having one, two or three or more carbon numbersspread is introduced through line 10 into dehydrogenator 11. A suitablestream may be for example, a 400 to 500F. boiling range n-paraffinfraction which includes l2, l3, and 14 carbon number paraffinhydrocarbons, i.e., dodecane, tridecane, and tetradecane. Indehydrogenator 11, the charge is contacted with a noble metal-molecularsieve catalyst at a temperature within the range of about 800 to 900F.,a pressure of 0 to 200'p.s.i.g. at a liquid hourly space velocity withinthe range of about 0.5 to 10 and in the presence of hydrogen in anamount within the range of about 0.5:1 to 15:1 moles of hydrogen permole of paraffin hydrocarbon. The resulting dehydrogenated product ispassed through line 12 to fractionator 13. In fractionator 13 theproducts are separated into fractions lighter boiling than the feedstockdischarged through line 15, heavier boiling than the feedstockdischarged through line 16 and hydrocarbons boiling within the range ofthe feedstock discharged through line 17. The lighter and heavierfractions in lines 15 and 16 may be further processed for separation ofthe corresponding olefins or may be discharged for other uses not shown.

The n-paraffin olefin mixture in line 17 is combined with solvent, forexample, a methylethyl ketonetoluene mixture from line 20 and passed tochiller 21. In chiller 21 the solvent diluted mixture is cooled to atemperature at which the n-paraffin constituents are precipitated. Theresulting slurry of waxlike n-paraffin crystals in the remaining liquidis passed through line 22 to filter 23. In filter 23 the crystals ofn-paraffins are separated and discharged through line 24 and remainingliquid comprising solvent and enriched olefins is discharged throughline 25. The n-paraffin stream is passed to stripping tower 29 whereindissolved solvent is removed through line 30 and n-paraffins withdrawnthrough line 31. The n-paraffins in line 31 are recycled to combine withthe charge in line 10.

The filtrate of enriched olefins in line 25 is passed to stripper 35wherein dissolved solvent is removed through line 36. The enrichedolefin product is withdrawn through line 37 and a portion or all of thisenriched olefin product is passed through line 38 to alkylator 39 asfeedstock for alkylation of benzene. Benzene feed is introduced intoalkylator 39 through line 40. Benzene and olefin stream are contactedwith a Friedel-crafts catalyst such as aluminum chloride-HG] at atemperature within the range of 90 to 140, at a pressure of aboutatmospheric and an olefin to aromatic feed ratio within the range ofabout 1:1 to 0.111. Effluent from alkylator 39 is passed through line 45to separator 46. Separator 46 may comprise decantation, distillation,solvent extraction or other means of separation of the alkylationproducts from the reactants which are well known in the art and whichforms no part of the present invention. Separated alkyl benzene productis withdrawn through line 47. Recovered n-paraffin hydrocarbons arewithdrawn through line 48 and may be recycled with the paraffinhydrocarbons in line 31 and the fresh charge in line to dehydrogenator11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In an example of the process ofthis invention, a charge stock consisting essentially of C and Cnparaffins and olefins was treated to produce an enriched olefinfraction. The charge stock contained 92.3 weight percent n-paraffins and7.7 weight percent olefins. The charge stock was diluted with 2 parts ofmethylethyl ketone and the resulting mixture chilled to 25F. A waxlikesolid precipitated which was separated by filtration at -25F. The waxysolid was washed with an additional two volumes of methylethyl ketone toremove occluded liquid and the wash solution added to the firstfiltrate. The filtrate and wash was stripped to remove solvent producingan olefin concentrate containing 21.0 weight percent olefins.

In another example, the C C, charge stock of the previous example wasdistilled separating separate C and C cuts. These fractions were alsoconcentrated by the same method. The yields of the C and C fractionsupon distillation were 54.3 and 45.7 weight percent respectively. Eachfraction was concentrated employing dilution with two volumes ofmethylethyl ketones and filtration at a temperature of 25F. Theresulting solid in each case was washed with an additional two volumesof solvent. Tests on each of the charge stocks and the resulting olefinconcentrates and residues are shown in the following tabulation:

It will be observed that the best results are obtained with essentiallysingle carbon number fractions as compared with fractions containing twoor more carbon numbers. Olefin enrichment was especially effective forthe C fraction where olefin content was increased five-fold, i.e., from7.8 to 39.7 weight percent. If desired, further enrichment may beeffected by repeating the enrichment step.

We claim:

1. A process of separating an olefin fraction from a mixture of olefinsand n-paraffins which comprises:

forming a solution of said mixture in a selective solvent,

cooling said solution to a temperature at which said n-paraffin aresolid and substantially insoluble in said solvent and at whichsubstantially all of said olefins remain in solution in said solventeffecting precipitation of n-paraffins from remaining solution ofincreased olefin concentration,

separating precipitated n-paraffins from said remaining liquid, and

separating solvent and said olefin fraction from said remaining liquid.

2. The process of claim 1 wherein said selective solvent is selectedfrom the group consisting of acetone, methylethyl ketone, methylisobutylketone, benzene, toluene naphtha, gasoline, pentane, dichloromethane,methylene chloride, chloroform, and mixtures thereof.

3. The process of claim 1 wherein said selective solvent comprises amixture of an aliphatic ketone selected from the groups consisting ofacetone, methylethyl ketone, methylisobutyl ketone, and mixtures thereofand an aromatic hydrocarbon selected from the group consisting ofbenzene, toluene, and mixtures thereof.

4. The process of claim 1 wherein said olefin fraction comprises 12 and13 carbon number olefins and solution is cooled to a temperature withinthe range of about 25F. effecting precipitation of said normal paraffinsas a waxy solid.

5. The process of claim 4 wherein said waxy solid is separated from saidremaining liquid by filtration at a temperature within the range ofabout 25F.

6. The process of claim 1 wherein said mixture of olefins andn-paraffins consists essentially of olefins and n-paraffins havingcarbon numbers differing by not more than two carbon numbers.

7. The process of claim 1 wherein said olefin fraction is contacted withbenzene in the presence of an alkylature of olefins and n-parafi'ms andsaid mixture of olefins and n-paraffins comprises a range ofhydrocarbons differing in carbon number by not more than two carbonnumbers.

2. The process of claim 1 wherein said selective solvent is selectedfrom the group consisting of acetone, methylethyl ketone, methylisobutylketone, benzene, toluene naphtha, gasoline, pentane, dichloromethane,methylene chloride, chloroform, and mixtures thereof.
 3. The process ofclaim 1 wherein said selective solvent comprises a mixture of analiphatic ketone selected from the groups consisting of acetone,methylethyl ketone, methylisobutyl ketone, and mixtures thereof and anaromatic hydrocarbon selected from the group consisting of benzene,toluene, and mixtures thereof.
 4. The process of claim 1 wherein saidolefin fraction comprises 12 and 13 carbon number olefins and solutionis cooled to a temperature within the range of about -25*F. effectingprecipitation of said normal paraffins as a waxy solid.
 5. The processof claim 4 wherein said waxy solid is separated from said remainingliquid by filtration at a temperature within the range of about -25*F.6. The process of claim 1 wherein said mixture of olefins andn-paraffins consists essentially of olefins and n-paraffins havingcarbon numbers differing by not more than two carbon numbers.
 7. Theprocess of claim 1 wherein said olefin fraction is contacted withbenzene in the presence of an alkylation catalyst under alkylationconditions effecting formation of alkyl benzene.
 8. The process of claim1 wherein an n-paraffin charge stock is dehydrogenated forming saidmixture of olefins and n-paraffins.
 9. The process of claim 8 whereinprecipitated n-paraffins are recycled to comprise a portion of saidn-paraffin charge stock.
 10. The process of claim 8 whereindehydrogenated n-paraffin charge stock is distilled separating saidmixture of olefins and n-paraffins and said mixture of olefins andn-paraffins comprises a range of hydrocarbons differing in carbon numberby not more than two carbon numbers.